Conventional image scanner generally employs keystroke or software programming for initiating scanning, without any function for initiating scanning automatically, thus the operation of the image scanner is rather tedious. Some image scanners have been designed for initiating scanning automatically, but the devices used, such as microswitch, photointerrupter, photoresistor, phototransistor, are expensive and are hard to achieve reliability, thus increasing the cost of the image scanner, making the manufacturing process more complicated, and lowering the reliability.